1. Field of the Invention
The present invention relates to a power supply and an image forming apparatus. In particular, the present invention relates to a flyback power supply circuit such as a ringing choke converter.
2. Description of the Related Art
In general, a switching power supply such as a flyback converter operating in a discontinuous mode is designed in such a manner that the oscillating frequency becomes minimum at a maximum load state required for the equipment in which the switching power supply is mounted, from a viewpoint of supplying power. Therefore, as the load becomes smaller in a sleep state or the like of the equipment, the oscillating frequency becomes higher. Further, when the oscillating frequency increases, an increase of electro magnetic interference (EMI) such as a noise terminal voltage, deterioration of power conversion efficiency, or other problems occur. In particular, the increase of the noise terminal voltage not only makes a design of a filter circuit for suppressing the noise more complicated but also increases the space occupied by the filter circuit in the apparatus itself, which inhibits downsizing of the equipment. As a measure for solving the problem, for example, Japanese Patent No. 3697218 describes a structure for lowering an operation frequency (oscillating frequency) of the ringing choke converter when the apparatus is in the sleep mode (in a light load state).
However, the related-art switching power supply proposed in Japanese Patent No. 3697218 has a problem in that the oscillating frequency is increased in a light load state of the equipment when an equipment sleep signal is not given. This problem is briefly described below with reference to FIGS. 1A and 1B. Note that, FIGS. 1A and 1B are described in detail in embodiments according to the present invention described later. As the flyback converter operating in the discontinuous mode, there are a switching power supply using diode rectification such as a ringing choke converter (RCC) illustrated in FIG. 1A and a switching power supply using synchronous rectification such as an RCC illustrated in FIG. 1B.
In the diode rectification RCC illustrated in FIG. 1A, a power loss occurs in a rectifying diode D802 on the secondary side due to a forward voltage drop of the rectifying diode D802. On the other hand, the synchronous rectification RCC illustrated in FIG. 1B performs the same operation as the diode rectification RCC while reducing the power loss in the rectifying diode D802 that is generated in the above-mentioned diode rectification RCC. The synchronous rectification RCC can reduce an ON-resistance of a switching element Q1101 in the conductive state so as to reduce a power loss generated in the switching element Q1101 in the conductive state to be smaller than the power loss generated in the diode D802 of the diode rectification RCC. Therefore, the synchronous rectification RCC can improve power conversion efficiency of the switching power supply than the diode rectification RCC.
However, the synchronous rectification RCC still has the problem in that the oscillating frequency is increased in a light load state, similarly to the diode rectification RCC. In addition, because the synchronous rectification RCC has the improved power conversion efficiency compared with the diode rectification RCC, the synchronous rectification RCC causes another problem in that the oscillating frequency is increased when supplying energy to the same load. Further, when the oscillating frequency is increased, noise is also increased.